1. Field of the Invention
The present invention relates to an image-reproducing apparatus for the line-wise exposure of a light-sensitive sheet placed on the surface of a drum, by means of a reproducing head revolving in the drum. These apparatus are also known as "internal drum" exposure apparatus.
2. Description of the Prior Art
A method is known of reproducing an image on a light-sensitive surface by scanning the surface with a light beam modulated in accordance with an electric signal representing the density of successive elements of the image, which method comprises the steps of: mounting the light-sensitive recording surface on the cylindrical surface of a stationary cylinder, mounting a rotatable prism having a plurality of internal reflecting surfaces, with a number of said surfaces sequentially reflecting an incident light beam, within and on the axis of said cylinder and rotating said reflector about the said axis; directing a light beam along the axis of said cylinder so that it is internally reflected by said number of surfaces internally to said cylindrical surface to define a circular track thereon; and progressively displacing said circular track along said cylindrical surface to define a scanning raster on the said light-sensitive surface mounted on said cylindrical surface by relative movement of said cylinder and light-sensitive surface on the one hand and said prism on the other hand in a direction parallel to the axis of said cylinder.
The mentioned method is disclosed in U.S. Pat. No. 3,875,587.
The displacment of the circular track along the cylindrical surface occurs in practice by axially displacing the rotatable prism in the cylinder by means of a carriage or a sliding table that is displaceable along a suitable rail system.
Known rail systems comprising ball guides or roller bearing guides cause vibrations if adjusted for zero play, and therefore are unacceptable in high resolution printing systems, such as in graphic printers, and printers used for the printing of medical diagnostic images, obtained by suitably processed digitized radiographic, ultrasonic, NMR or other signals.
Air bearings require a source of pressurized air for their operation, whereas a lack of stiffness may cause problems with the accuracy of reproduction. Slide bearings are capable of offering smooth and vibration-free transports, but to that end they require precision-ground and precision-polished metal slides or rods which inevitably are expensive.